1. Field of the Invention
This invention relates to an etching method particularly for forming a wiring pattern, a barrier metal or the like made of tungsten or its alloy.
2. Description of the Related Art
For example, a thin film resistor made of CrSi or the like adopts a sandwich structure as an electrode structure in which a barrier metal is sandwiched between an Al thin film and a thin film resistor to prevent mutual diffusion therebetween. Accordingly, its characteristics is prevented from deteriorating due to the mutual diffusion.
When such a barrier metal is formed by dry-etching which cannot provide a sufficient selective ratio between the barrier metal and the thin film resistor, the thin film resistor exposed by over-etching is easily etched. Generally, since the thin film resistor has a thin thickness of approximately 10 nm, the etching of the thin film resistor greatly changes characteristics such as a value of resistance thereof. Therefore, the barrier metal needs to be formed by wet-etching which can provide a sufficient etching selective ratio.
The wet-etching of the barrier metal, however, may produce residues which can interrupt the patterning of the thin film resistor and cause short-circuit to a wiring pattern. These problems are more specifically explained referring to FIGS. 17A to 17C, and 18A to 18E which show patterning processes of the sandwich structure composed of the Al thin film, the barrier metal, and the thin film resistor. The patterning process shown in FIGS. 17A to 17C are different from that shown in FIGS. 18A to 18E. FIGS. 17A to 17C shows a case where the thin film resistor is patterned after the barrier metal is patterned, while FIGS. 18A to 18E shows a case where the thin film resistor is patterned before the barrier metal is patterned.
When the process shown in FIGS. 17A to 17C is adopted, first, as shown in FIG. 17A, a CrSi film 102 for forming the thin film resistor and a TiW film 103 for forming the barrier metal are deposited on an insulation film 101 in sequence. After that, as shown in FIG. 17B, the TiW film 103 is patterned using a resist 104 as a mask to form the barrier metal. At that time, a residue 103a of TiW remains on the CrSi film 102. As shown in FIG. 17C, when the thin film resistor is patterned by, for example, chemical dry-etching to have a defined shape, the residue 103a adversely affects the patterning.
Meanwhile, when the process shown in FIGS. 18A to 18E is adopted, first, as shown in FIG. 18A, a 1st Al 202 is formed on an insulation film 201, and a thin film resistor 204 is formed adjacently to the 1st Al 202 through an insulation film 203. After that, as shown in FIG. 18B, a TiW film 205 for forming the barrier metal and an Al thin film 206 are deposited in sequence. As shown in FIG. 18C, the Al thin film layer 206 is patterned using a resist 207 as a mask. Further, as shown in FIG. 18D, the TiW film 205 is patterned to form the barrier metal. At that time, a residue 205a of TiW remains on the thin film resistor 204 and on the insulation film 203. Therefore, as shown in FIG. 18E, when a wiring pattern 209 is formed through an insulation film 208, the wiring pattern 209 may be short-circuited by the residue 205a.
It is not easy to remove the residues 103a, 105a by lengthening the etching time and/or by changing etching conditions such as a temperature of etching solution. For example, when the etching time is lengthened, the resist as the patterning mask is separated to deteriorate a patterning accuracy.